Electrode catheters have been in common use in medical practice for many years. They are used to stimulate and map electrical activity in the heart and to ablate sites of aberrant electrical activity.
In use, the electrode catheter is inserted into a major vein or artery, e.g., femoral artery, and then guided into the chamber of the heart which is of concern. Within the heart, the ability to control the exact position and orientation of the catheter tip is critical and largely determines how useful the catheter is.
In certain applications, it is desirable to have the ability to inject and/or withdraw fluid through the catheter. This is accomplished by means of an irrigated tip catheter. One such application is a cardiac ablation procedure for creating lesions which interrupt errant electrical pathways in the heart.
A typical ablation procedure involves the insertion of a catheter having a tip electrode at its distal end into a heart chamber. A reference electrode is provided, generally taped to the skin of the patient. RF (radio frequency) current is applied to the tip electrode, and current flows through the media that surrounds it, i.e., blood and tissue, toward the reference electrode. The distribution of current depends on the amount of electrode surface in contact with the tissue as compared to blood, which has a higher conductivity than the tissue. Heating of the tissue occurs due to its electrical resistance. The tissue is heated sufficiently to cause cellular destruction in the cardiac tissue resulting in formation of a lesion within the cardiac tissue which is electrically non-conductive. During this process, heating of the electrode also occurs as a result of conduction from the heated tissue to the electrode itself. If the electrode temperature becomes sufficiently high, possibly above 60.degree. C., a thin transparent coating of dehydrated blood protein can form on the surface of the electrode. If the temperature continues to rise, this dehydrated layer can become progressively thicker resulting in blood coagulation on the electrode surface. Because dehydrated biological material has a higher electrical resistance than endocardial tissue, impedance to the flow of electrical energy into the tissue also increases. If the impedance increases sufficiently, an impedance rise occurs and the catheter must be removed from the body and the tip electrode cleaned.
In a typical application of RF current to the endocardium, circulating blood provides some cooling of the ablation electrode. However, there is typically a stagnant area between the electrode and tissue which is susceptible to the formation of dehydrated proteins and coagulum. As power and/or ablation time increases, the likelihood of an impedance rise also increases. As a result of this process, there has been a natural upper bound on the amount of energy which can be delivered to cardiac tissue and therefore the size of RF lesions. Historically, RF lesions have been hemispherical in shape with maximum lesion dimensions of approximately 6 mm in diameter and 3 to 5 mm in depth.
In clinical practice, it is desirable to reduce or eliminate impedance rises and, for certain cardiac arrhythmias, to create larger lesions. One method for accomplishing this is to monitor the temperature of the ablation electrode and to control the RF current delivered to the ablation electrode based on this temperature. If the temperature rises above a preselected value, the current is reduced until the temperature drops below this value. This method has reduced the number of impedance rises during cardiac ablations but has not significantly increased lesion dimensions. The results are not significantly different because this method still relies on the cooling effect of the blood which is dependent on location in the heart and orientation of the catheter to endocardial surface.
Another method is to irrigate the ablation electrode, e.g., with physiologic saline at room temperature, to actively cool the ablation electrode instead of relying on the more passive physiological cooling of the blood. Because the strength of the RF current is no longer limited by the interface temperature, current can be increased. This results in lesions which tend to be larger and more spherical, usually measuring about 10 to 12 mm.
The clinical effectiveness of irrigating the ablation electrode is dependent upon the distribution of flow within the electrode structure and the rate of irrigation flow through the tip. Effectiveness is achieved by reducing the overall electrode temperature and eliminating hot spots in the ablation electrode which can initiate coagulum formation. More channels and higher flows are more effective in reducing overall temperature and temperature variations, i.e., hot spots. The coolant flow rate must be balanced against the amount of fluid that can be injected into a patient and the increased clinical load required to monitor and possibly refill the injection devices during a procedure. In addition to irrigation flow during ablation, a maintenance flow, typically at a lower flow rate, is required throughout the procedure to prevent backflow of blood flow into the coolant passages. Thus reducing coolant flow by utilizing it as efficiently as possible is a desirable design objective.
In view of the foregoing, accurate and real-time temperature measurement at a catheter tip providing actual interfacial temperature is desirable. Typical temperature sensors for use with catheters can be up to 30 degrees off from the actual tissue temperature. An ablation catheter with improved temperature sensing capabilities should prevent thrombus formation and tissue charring. It would also provide better tissue/blood contact interface temperature reading allowing an operator better power control. Improved temperature measurement would also have applications to other catheter-based technologies, such as esophagus, VT and other applications where tissue monitoring is a key measurement at a catheter tip.
For improved sensing capabilities, Micro-Electro-Mechanical Systems (MEMS) offer the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through microfabrication technology. MEMS components are typically made using microfabrication processes that can form very thin layers, and compatible “micromachining” processes that selectively etch away parts of a silicon wafer or add new structural layers to form mechanical and electromechanical devices.
Sensor technology that can be integrated into semiconductor materials for sensing characteristics including temperature are well known in the art. A temperature gauge can be constructed using a resistor made of a material such as polysilicon, or other thermoresistive material, whose resistance changes with temperature. Using this type of a sensor, temperature can be measured as a function of the change in the resistance of the material. Furthermore, a temperature gauge can also be constructed by forming a thin film thermocouple.
Accordingly, there exists a need for a catheter with improved temperature sensing capabilities, including an ablation catheter with a microfabricated temperature sensor positioned on the outer surface at or near the distal tip for real time, actual interfacial temperature measurement.